The present invention relates generally to magnetic resonance imaging, and more specifically, to a system and method for mapping the sensitivity of radio frequency (RF) coils of a magnetic resonance (MR) system. The invention finds particular utility in parallel imaging applications by providing quickly determined coil sensitivity maps, requiring fewer flip angles/scans to determine.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Often, it is desirable in MR imaging procedures to map the strength and variations in a B1 magnetic field across the desired field of view, to decrease the potential effects on image reconstruction, or to allow for tailored excitation profiles across the field of view by manipulating individual currents in surface coils. Normalization or compensation functions can be generated from B1 maps and applied to the received NMR signals to account for magnetic field in-homogeneities. The same B1 maps can be used to extract information for driving the currents in individual RF coils in the transmit mode to achieve tailored excitations across the field of view. These mappings are usually performed for each scan subject, as subject positioning within the coil and subject-specific local physical properties (such as dielectric constant) can influence the coil B1 homogeneity.
Methods for mapping the sensitivity of RF coils, or alternatively the uniformity of the B1 field, typically require that multiple data acquisitions take place for each coil of an RF coil assembly, where each data acquisition is taken at a different flip angle. This typically results in the use of 2-8 different flip angles or transmit powers for each coil of an N-channel coil array. Therefore, a total of 2N-8N different acquisitions are often used to generate one sensitivity mapping. The signals acquired in these acquisitions are proportional to the sine of the flip angle. Therefore, a pixel by pixel fit of the intensity of the signal versus the flip angle (from the known RF pulse height or width) can produce a map of the coil sensitivity. This method of coil sensitivity mapping is well known in the art. However, as the number of calibration or sensitivity-measuring scans required to produce this pixel by pixel fit is increased, overall scan time and patient throughput decrease.
One particular application where coil sensitivity mapping is particularly advantageous is parallel imaging. Usually the system of spins is excited using a homogeneous coil (such as a whole-body RF coil), and then an array of surface RF coils is used to receive the MR signals, in order to increase the step size between phase-encoding lines, or equivalently to reduce the size of the field of view and the amount of data collected. Scan time reduction is achieved by under-sampling k-space and recording images simultaneously from the multiple imaging or receive coils. Under-sampling generally reduces the data acquisition time by increasing the distance of sampling positions in k-space.
Parallel imaging techniques not only expedite data acquisition, but also reduce aliasing or wrapping that occurs in the phase-encoding direction when an imaging object extends outside a field-of-view (FOV). In particular, parallel imaging techniques remove or reduce the aliasing by using surface coil B1 fields (sensitivities), to define or determine an un-aliased spin distribution. Information regarding the surface coil B1 fields or sensitivities is typically acquired with an external calibration or a self-calibration technique. Generally, the coil sensitivity data is used to weight the imaging data such that coil sensitivity is reflected in the reconstructed image, and, as a result, the coil sensitivity data reduces aliasing in the reconstructed image that can occur as a result of under-sampling.
In a manner similar to that previously described, an array of RF coils can also be used to both transmit and receive the MR signals (as opposed to using a homogenous RF coil for transmitting and many surface coils for receiving the signal). Such an approach has been shown to be useful for RF shimming, pulse designs for small FOV imaging, or reducing specific absorption rate for certain other MR applications. For these applications, methods of pulse design typically rely on knowledge of the particular excitation and reception pattern of each of the small RF coils employed.
In all these applications, the time spent determining coil sensitivity should be as little as possible. It would therefore be desirable to have a system and method capable of quickly producing coil sensitivity maps while requiring a reduced number of mapping acquisitions.